Six leaders from South Korea's largest Buddhist order have resigned after secret video footage showed the monks playing high-stakes poker, drinking and smoking.

The scandal broke just days before Koreans observe a national holiday to celebrate the birth of Buddha, the holiest day of the religion's calendar.

The head of the Jogye order, which has 10 million followers – about a fifth of the population – made a public apology on Friday, vowing "self-repentance".

South Korean TV networks aired shots of monks playing poker, smoking and drinking, after gathering at a luxury lakeside hotel in late April for a fellow monk's memorial service. "The stakes for 13 hours of gambling were more than 1bn won [£543,000]," Seongho, a senior monk, told Reuters on Friday. He said he had reported the incident to prosecutors.

Gambling is illegal in South Korea outside of licensed casinos and horse racing tracks and is frowned upon by religious leaders.

"Buddhist rules say don't steal. Look at what they did, they abused money from Buddhists for gambling," Seongho said.

Seongho said he had obtained a computer memory stick with the video clip from a camera that had been hidden in the hotel. He would not say who had planted the camera because of recent threats made against him.

The scandal has cast doubt on the future of the order's head, Jaseung, who apologised to all of South Korea's 12 million Buddhists.

"We deeply apologise for the behaviour of several monks in our order," he said in a statement. "The monks who have caused public concern are currently being investigated and will be punished according to Buddhist regulations as soon as the truth is verified by the prosecution."

Chung Yoon-sun, the secretary general of the Buddhist Solidarity for Reform, said conflict between South Korean monks had become as commonplace as disputes between the country's politicians. "It's just like politics," she was quoted as saying by the Korea Times. "If there's a conflict in interest between two groups, they make a deal or they fight."

Chung said the scandal highlighted the need to monitor how Buddhist orders spend their large, and untaxed, donations from the public.